AC plasma display panels or gas discharge panels are well known in the art and generally comprise a pair of glass substrates respectively supporting column and row electrode patterns. The electrode patterns are coated with dielectric layers and are disposed in parallel spaced relation to define a gap therebetween. The substrates are arranged such that the electrode patterns are disposed in orthogonal relation and thereby define points of electrode intersection which, in turn, define discharge cells. A dischargeable gas is positioned in the gap between the dielectric layers and may be selectively discharged by appropriate energization of intersecting electrodes.
Such panels operate with AC voltages and provide a write voltage which exceeds the firing voltage of the gas at a given discharge "pixel cell" as defined by selected, intersecting, column and row electrodes. A discharge at a selected pixel cell can be continuously "sustained" by applying an alternating polarity sustain voltage, which by itself is insufficient to initiate a discharge. The sustain action relies upon wall charges which are generated on the dielectric layers and cooperate with the applied sustain signal to exceed the gas breakdown voltage.
Early versions of the AC plasma display panel applied both addressing and sustain signals to common electrodes (see U.S. Pat. No. 3,559,190 to Bitzer et al.). Later, improved AC plasma displays have separated the sustain and address circuitry to achieve greater flexibility in operation and lower drive circuit costs (see U.S. Pat. No. 4,772,884 to Weber et al.). In most AC plasma display structures, all electrodes within a display area of the panel structure are covered by the dielectric layer, thereby enabling wall charge accumulation and preventing arc-over which could occur if conductors were directly exposed to the ionizable gas.
DC-operated plasma display panels employ DC electrodes that are in direct contact with the gas, but employ current limiting devices in the drive circuitry to prevent excessive current flow when the gas discharges. To confine the discharge area within a DC plasma display panel, dielectric separators are positioned between the pixel cells and prevent the spread of the ionized gas. In AC plasma panels, such separators are not required.
DC electrodes have also been used in AC plasma panel structures. Engineers at the Fujitsu Corporation have developed a panel that has dielectric-covered AC electrodes on one substrate and orthogonally oriented, metal DC electrodes on the other substrate. This panel is described in a paper entitled "Improvement of Luminance and Luminous Efficiency of Surface-Discharge Color AC PDP" by Shinoda et al., Society for Information Display, International Symposium, Digest of Technical Papers, vol. 22, pages 724-727, May 6-10, 1991. In the Shinoda et al. panel, AC electrodes on the front glass plate were energized with the usual AC sustain signal and DC electrodes on the rear glass substrate were used only for addressing.
It has recently been found that AC plasma display panels exhibit a reliability problem that is manifested by an irreversible breakdown of the thick film dielectric layer over the electrode structures. The dielectric layer is the fundamental insulating layer that prevents destructive arc discharge in the panel. Referring to FIG. 1, a cross-section of a prior art AC plasma panel is shown and includes glass substrates 10 and 12 that enclose a dischargeable gas 14. Row electrodes 16 reside on the lower surface of glass substrate 10 and column electrodes 18 (only one is shown), reside on the upper surface of glass substrate 12. Electrodes 16 and 18 are each covered by dielectric layers 20 and 22 which may, in turn, be covered by a further dielectric MgO overcoat (not shown).
Dielectric layers 20 and 22 are normally designed to withstand voltages of at least 500 volts. Typically used sustain pulses are approximately 100 volts and, when added to a maximum value of 100 volts of wall potential which appear on the walls of dielectric layers 20, 22, result in a maximum potential across both dielectric layers 20 and 22 of approximately 200 volts (or 100 volts across each dielectric). Thus the 500 volt breakdown strength for each dielectric layer offers a significant safety factor under normal operation. Nevertheless, it has been found that under certain circumstances, dielectric layers 20 and 22 experience catastrophic breakdown, with a resultant arcing between electrodes 16 and 18. Once this breakdown has occurred, the panel is permanently damaged and in the neighborhood of the damaged dielectric, will not function properly. The cause of such dielectric breakdown has heretofore been thought to result from dielectric abnormalities or deficiencies. As will be apparent from the below described invention, the Applicant has found that the cause of the dielectric breakdown is entirely different than heretofore understood.
Accordingly, it is an object of this invention to provide an improved method for operating an AC plasma panel which prevents arc breakdown of dielectric layers covering panel electrodes.
It is yet another object of this invention to provide an improved AC plasma panel structure which prevents conditions from occurring within the AC plasma panel that lead to dielectric layer breakdown.